1. Field of the Invention
The present invention relates to a method for recovering zinc from zinc oxide bearing material containing silicates. According to the method, the leaching of zinc compounds is carried out in circumstances where the silicates remain undissolved and therefore do not cause filtering problems. The silicate bearing residue is treated in a Waelz process, so that the major part of the silicic acid of the silicates is combined to the iron silicate slag formed in the Waelz process, and the zinc is returned to leaching, mainly in oxidic form.
2. Description of the Preferred Art
Zinc sulfide concentrate forms the major part of the raw materials in the electrolytic zinc process. The concentrate is roasted, and the formed calcine is leached. In addition to zinc, the concentrate contains remarkable amounts of iron, as well as quartz, among others. During roasting, iron produces zinc ferrite ZnO.Fe.sub.2 O.sub.3, and quartz produces zinc silicates Zn.sub.2 SiO.sub.4 ; these are important bearers of zinc, apart from the principal component, zinc oxide.
Generally the zinc oxide of the zinc calcine is first leached with weak sulfuric acid (electrolysis return acid), in neutral leaching comprising either one or two stages. After solution purification, the formed zinc sulfate solution is conducted to electrolysis. The undissolved ferrites require their own special treatment, which is generally carried out hydrowise, so that they are conducted either to strong acid leaching or to a conversion stage described for instance in the Norwegian patent 108,047 and in the U.S. Pat. No. 3,959,437. In the strong acid leaching and the conversion stage, the pH remains so low (&lt;2), that in addition to ferrites, also the zinc silicate is dissolved. The major part of the dissolved iron is precipitated as jarosite or goethite, and the silicic acid of zinc silicate is precipitated as SiO.sub.2. The formed iron--silicon acid slag is washed and transported to a disposal site, and the solution, containing the zinc, is returned to neutral leaching. The returned solution also contains a certain amount of ferric iron, which is essentially important in the removal of certain hazardous materials, such as antimony, arsenic and germanium, in the neutral leaching stage.
In order to recover the zinc, the finely divided ferritic leaching residue obtained from the neutral leaching of the calcine may also be conducted to a Waelz process, where the precipitate is treated by means of coal at a high temperature in such a reducing fashion, that the remaining zinc is evaporated, and at the same time the iron is combined to the iron silicate slag. The evaporated zinc is oxidized into zinc oxide, which is fed back to neutral leaching. The formed zinc oxide is not absolutely pure, but contains a small amount of silicate, too. When this is transferred back to circulation, dissolving of the silicate again causes problems. The settling and filtering of a silicate gel formed in acid leaching requires several stages, and a large amount of washing water is needed. When the washing of the precipitate remains insufficient, into the Waelz furnace there is fed zinc sulfate containing precipitate, the drying whereof requires energy, and reduction on the other hand requires a lot of coal. The sulfur formed in decomposing of the sulfate goes to the gases as sulfur dioxide, and is thus hazardous for the environment.